The invention relates to acoustic emissions testing of piping, particularly buried piping which is not readily accessible. Specifically, the invention relates to a self-contained probe which permits acoustic emission testing to proceed more rapidly and accurately than has heretofore been possible.
Acoustic emission testing has been developed in recent years, particularly as a means of testing pressure vessels for flaws which could cause mechanical failure. The method as applied to pressure vessels is described in U.S. Pat. No. 3,545,262 and in many articles available in the literature.
Methods of testing buried pipes for leaks have been described in many prior art patents. In particular, the following are noted: U.S. Pat. Nos. 1,881,543; 2,008,934; 3,028,450; 3,055,209; 3,170,152; 3,261,200; 3,289,465; 3,458,656; 3,478,576; 3,508,433; 3,575,040; 3,673,857; 3,691,819. Many of these techniques relied upon the emission of acoustic energy when a fluid contained within the pipe passed through a leak in the wall. These methods differ substantially from that of the present invention in that they generally rely on the measurement of sounds in relatively lower frequency ranges, typically 2HZ to 20 KHZ, created by the passage of fluid through a leak. Acoustic emission testing, as the term is used in the art, is primarily concerned with detecting flaws, not necessarily those which are large enough to represent leaks, but flaws which might cause failure of the pipe when subjected to stress in service or which could lead to leaks. Acoustic emission testing relies, not on the sound produced by fluid passing through a leak, but rather upon sound emitted by the metal itself when subjected to an increasing stress, it being characteristic of such flaws that they cause intermittent bursts of acoustic energy as stress is applied. In contrast to the prior art leak testing, frequencies measured in acoustic emission testing are typically in the range of 25 KHZ to 1 mHZ.
When such a technique is applied to buried piping, considerable difficulties are encountered in the placement of the transducers which are used to receive and properly position, by computer analysis, the location of the flaws contained within the pipe. It will be appreciated that since piping may well be a mile or more in length, it is difficult to properly locate the flaws. Since the acoustic energy is attenuated by passage through the metal, it is necessary to place transducers relatively close together. Generally, a spacing of 200-800 feet is reasonable. With buried piping, positioning of transducers is quite difficult since excavation is required at each point of contact with the pipe, the pipe's surface must be carefully prepared, and transducers must be attached securely in order to accurately receive the acoustic energy transmitted through the pipe. The present invention overcomes the major portion of these difficulties and permits rapid analysis of buried piping for leaks and flaws while eliminating much of the expensive and time-consuming activity heretofore required.